Blood cholesterol is a major risk factor for coronary heart disease (CHD) due to the central role that cholesterol metabolism plays in the disease. Circulating low density lipoprotein (LDL) is modified through oxidation in vascular tissue. Macrophages in the subendothelial space take up these cholesterol-rich particles and accumulate large quantities of cholesteryl esters and pro-inflammatory lipids, thereby becoming foam cells. This sets up a chronic inflammatory process in the arterial wall involving macrophages, other inflammatory cells, cytokines, and smooth muscle cells, followed by disruption of the arterial endothelial surface, vessel narrowing, and ultimately thrombosis and vessel occlusion resulting in myocardial infarction (MI). Lowering circulating LDL decreases the uptake of oxidized LDL by macrophages, thereby limiting this pathological process. Statins decrease risk for CHD by lowering LDL. In addition to cholesterol delivery mechanisms mediated by LDL, there is a cholesterol removal process termed reverse cholesterol transport (RCT) in which excess cholesterol in peripheral tissues is trafficked back to the liver where it is secreted via the bile into the intestine, and excreted in feces (Cuchel, M., Rader, D. J. (2006). Circulation 113(21): 2548-2555). The transfer of cholesterol to the liver takes place, in part, on high density lipoprotein (HDL) and this key role in RCT is one of the main reasons why HDL cholesterol (HDL-C) is often the lipid risk factor most closely correlated with CHD in epidemiologic studies. i.e. HDL-C has a strong inverse correlation with disease risk (Gordon, D. J. et al., (1989). Circulation 79(1):8-15; Duffy D. & Rader D. J. (2009). Nat Rev Cardiol 6 (7):455-63.). In addition, HDL has anti-oxidant activity that inhibits the generation of pro-inflammatory oxidized LDL.
The removal of cholesterol from atherosclerotic lesions is thought to attenuate the disease process and, thus, stimulating RCT is likely to be a beneficial therapeutic modality. This is a major rationale for developing LXR agonists for the treatment of atherosclerosis. LXRs (α and β isoforms) are master regulators of cellular and whole-body RCT controlling the transcription of genes involved in all major phases of movement of peripheral cellular cholesterol to the liver and out of the body.
LXRs are able to sense excess intracellular cholesterol by binding to and being transactivated by specific oxysterol cholesterol metabolites. Upon activation, LXRs induce the expression of a variety of cholesterol efflux transporters, apolipoproteins, and lipoprotein modification pathways in multiple tissues that facilitate the removal of excess cellular and whole-body cholesterol (Fiévet C, Staels B. (2009). Biochem Pharmacol. 77(8): 1316-27). It is anticipated that such an integrated stimulus of foam cell macrophage cholesterol efflux, trafficking in the circulation, uptake and metabolism in the liver, and excretion in feces will have a robust anti-atherosclerosis effect.
Two important target genes that are induced by LXR agonists in a variety of tissues, including foam cell macrophages, are the ABC transporters ABCA1 and ABCG1. These are lipid efflux transporters that pump cholesterol out of the cell onto HDL acceptors, generating HDL-C. They play a critical role in helping macrophage foam cells efflux excess sterol (Jessup, W., I. C. Gelissen, et al. (2006). Curr Opin Lipidol 17(3): 247-57). LXR agonists also induce apolipoprotein E in macrophages (Laffitte, B. A., J. J. Repa, et al. (2001). Proc Natl Acad Sci USA. 98(2): 507-12.), which also helps to promote cholesterol efflux from these cells. HDL-C can be taken up directly by the liver or the cholesterol can be first transferred to LDL via the cholesteryl ester transfer protein (CETP) and be delivered to the liver through the LDL receptor. LXRs also induce CETP expression in liver and adipose tissue (Luo, Y. and A. R. Tall (2000). J Clin Invest. 105(4): 513-20.), which could facilitate RCT flux via the LDL pathway. Excess hepatic cholesterol can be converted to bile acids or secreted directly into the bile for subsequent excretion. The liver secretion and intestinal excretion steps are also stimulated by LXR agonists through the induction of two additional ABC transporters, ABCG5 and ABCG8 (Repa, J. J., K. E. Berge, et al. (2002). J Biol Chem. 277(21): 18793-800.). These transporters pump cholesterol out of the hepatocyte into bile and also limit absorption of cholesterol by transporting enterocyte cholesterol into the lumen of the gut.
LXRs also inhibit the NF-κB-dependent induction in macrophages of a variety of inflammatory genes such as iNOS, COX-2 and IL-6 among others (Joseph, S. B., A. Castrillo, et al. (2003). Nat Med. 9(2): 213-9.), and LXR agonists inhibit inflammatory processes in vitro and in vivo. Recent studies also suggest that synthetic LXR agonists could affect acquired immunity by limiting T-cell proliferative responses to activating signals (Bensinger, S. J., M. N. Bradley, et al. (2008). Cell 134(1), 97-111.). These effects on innate and acquired immunity are additional potential anti-atherosclerotic mechanisms of LXR agonists.
LXRs also have favorable effects on glucose homeostasis. Treatment of diabetic mouse models with LXR agonists results in the inhibition of hepatic PGC-1, PEPCK, and glucose-6 phosphatase (G6Pase) and the stimulation of hepatic glucokinase, resulting in marked inhibition of hepatic glucose output (HGO) (Laffitte, B. A., L. C. Chao, et al. (2003). Proc Natl Acad Sci USA. 100(9): 5419-24.). In addition, GLUT4 expression in adipose tissue is upregulated by LXR agonism, thereby increasing peripheral glucose disposal. Consistent with this, LXR agonist treatment of cultured adipocytes increased glucose uptake. Finally, LXR agonism appears to downregulate glucocorticoid action in liver. LXR agonists inhibit hepatic 11β-HSD1 expression (Stulnig, T. M., U. Oppermann, et al. (2002). Diabetes. 51(8): 2426-33.), an enzyme that converts inactive cortisone to active corticosterone, thus likely lowering liver glucocorticoid. This downregulation of hepatic glucocorticoid activity is likely the mechanism for LXR regulation of PEPCK, G-6-Pase, and glucokinase. Thus, by both inhibiting hepatic glucose output and stimulating peripheral glucose disposal, LXR treatment markedly lowers plasma glucose in diabetic rodent models.
Recently LXRs have also been shown to be important regulators of prostate cancer cell survival. Disruption of lipid rafts in response to LXR-dependent cholesterol efflux (Dufour J. et al. (2012). Curr Opin Pharmacol. 2012 Jul. 19). Lowering membrane cholesterol results in a suppression of the AKT survival pathway and consequently apoptosis. Thus, stimulating the LXR-AKT pathway may be beneficial for prostate cancer. Similarly, LXR activation has been suggested to have utility in treating a variety of other cancers including those of the breast (Vedin, L-L. et al., (2009) Carcinogenesis. 30 (4): 575-79) and pancreas (Rasheed et al., (2012) Cancer Research. 72 (8), Supplement 1, Abstract 3494).
LXR agonists have also been suggested to be useful for the prevention and treatment of photo and chronological skin aging, through their positive effects on keratinocyte and fibroblast gene expression (Chang, K. C. et al., (2008) Mol Endocrinol. 22(11): 2407-19).
In addition to the positive effects on cholesterol metabolism, LXRs stimulate fatty acid and triglyceride (TG) synthesis in the liver, primarily through inducing the transcription factor SREBP-1c. Consequently, most LXR agonists cause at least some degree of undesirable accumulation of lipids within hepatocytes and elevated plasma TG and LDL (Groot, P. H., et al. (2005). J Lipid Res. 46(10): 2182-91), a property primarily attributed to LXRα specific activity (Peet, D. J., et al. (1998). Cell. 93(5): 693-704; Lund, E. G., et al. (2006). Biochem Pharmacol. 71(4): 453-63). This is the major mechanism-based adverse effect of the target class and is most pronounced in full pan agonists. Strategies to minimize the undesirable lipid effects include identifying LXRβ selective compounds that are also partial agonists. Partial agonists can display tissue-specific activation or repression of nuclear receptors (Albers, M., et al. (2006). J Biol Chem. 281(8): 4920-30), as was demonstrated for the anti-estrogen tamoxifen, which functions as an antagonist of estrogen signaling in breast tissue and an agonist in the uterus (Delmas, P. D., et al. (1997). N Engl J Med 337(23): 1641-1647). Characterization of LXR isoform-specific null mice indicates that LXRα is the predominant mediator of LXR activity in the liver Peet, D. J., et al. (1998). Cell. 93(5): 693-704; Lund, E. G., et al. (2006). Biochem Pharmacol. 71(4): 453-63). In macrophages, however, LXRβ alone is sufficient to mediate the effects of LXR ligands on target gene expression. Therefore, compounds with limited LXRα activity should have anti-atherogenic activity while limiting unwanted hepatic effects.
Liver X Receptors
LXRs are adopted orphan members of the nuclear receptor superfamily. There are two LXR isoforms, LXRα and LXRβ, and both heterodimerize with the Retinoid X Receptor (RXR) (Song, C., et al. (1994). Proc Natl Acad Sci USA. 91(23): 10809-13; Apfel, R., et al. (1994). Mol Cell Biol. 14(10): 7025-35; Willy, P. J., et al. (1995). Genes Dev. 9(9): 1033-45.). Both LXRs, when complexed with RXR, bind to distinct regions of DNA called LXR response elements (LXREs) present in the promoters of LXR target genes. The LXR response elements take the form of two degenerate hexad direct repeat sequences, the consensus being AGGTCA, separated by 4 nucleotides, collectively termed a DR4 repeat (Willy, P. J. and D. J. Mangelsdorf (1997). Genes Dev. 11(3): 289-98). LXRα is found predominantly in the liver, with lower levels found in kidney, intestine, spleen and adrenal tissue (see, e.g., Willy, et al. (1995) Gene Dev. 9(9):1033-1045). LXRβ is ubiquitous in mammals and was found in nearly all tissues examined.